Synthetic blood vessel grafts



United States Patent Glens Falls, N.Y., assignors to United States Catheter & Instrument Corporation, Glens Fails, N.Y., a corporation of New York No Drawing. Filed July 27, 1961, Ser. No. 127,114

' 25 Claims. (Cl. 117-62.2)

This invention relates to new and useful improvements in artificial blood vessel grafts and is more particularly concerned with fabric grafts that are sealed against blood leakage with cross-linked thiolated gelatin on implantation but are sutliciently porous thereafter to promote healing and insure satisfactory tissue growth through the fabric wall.

Artificial blood vessels that are knitted, woven or braided of various synthetic filaments such as Dacron (Du Pont trademark for polyethylene glycol terephthalate) or preferably Teflon (Du Pont trademark for tetrafluoroethylene resin) have now become a [fairly commonly used item in surgery.

A woven graft can be tightly woven so that the porosity is minimal and thus avoid leakage when first implanted. However, such a graft is not 'fnay resistant at its out edges and thus oifers more diflioulty in suturing but even more important, these tightly woven grafts appear to heal slower and do not develop a natural intima as quickly as more porous grafts.

A knitted graft, for example, cannot be knitted tightly enough to avoid leakage when first implanted but apparently both (heals and is provided with an intima quicker because of the porosity between the filaments and is sutured more effectively near the edges as there is no rfray-ing.

Thus, the ideal artificial artery would be one with a fabric formation adapted to effective suturing, and with a low porosity initially to avoid lealoage but suflicient porosity fairly soon thereafter to permit subsequent tissue growth through the artery wall to encourage intima formation and anchorage.

It has been suggested to fill or coat the walls with gelatin which will serve the immediate purpose of preventing leakage but be subsequently absorbed by the blood stream to ultimately leave a more porous wall. However, gelatin is difficult to treat (denature) so that it may be autoclaved (for sterilization) and even after denaturing, the film on drying is stiff and subject to cracking, the first of which is inconvenient and the second of which breaks the seal and may be fatal to a patient if particles of gelatin are dislodged. Moreover, autoclaving of denatured gelatin films tends to make the gelatin water-soluble again. Furthermore, at its best, the dipping of fabric arteries in conventional gelatin tends to leave bubbles which create difliculties as they must be eliminated and requires a four hour bake to denature the gelatin.

It is an object of this invention to provide artificial fabric arteries that are sealed against leakage on placement with a sealing material that is removed thereafter to leave the natural porous structure of the fabric for penetration by tissue to form the naturalintima and heal the graft in its position.

It is an additional object of this invention to provide such arteries wherein the sealing material is gradually digested by enzymes in the blood.

It is a further object of this invention to provide such arteries that may be readily autoclaved for purposes of sterility.

It is another object of this invention to provide such arteries that are soft, pliable and otherwise suitable for surgical implantation and offer no risk of cracking or flaking.

It is also an object of this invention to provide such arteries that may be coated in much less time than with conventional denatured gelatin.

It is a du-rther object of this invention to provide a novel gelatin product that may be utilized in various forms in animal bodies because of its initial insolubility but which will subsequently be digested by body fluids.

We have found that fabric tubes, can be coated with a solution of thiolated gelatin and the thiolated gelatin oxidized to a heat-stable, unmeltable gelatin in situ on the fabric tube to provide an artificial artery that will not leak when surgically implanted but which will permit subsequent penetration of the :fiabric pores by infiltrating tissues as the thiolated and oxidized gelatin is gradually absorbed and digested by enzymatic action of the blood in the same manner as conventional or denatured gelatin. Hatch grafting fabrics can be desirably coated in the same manner. In addition, we have found that thiolated and oxidized gelatin may be tanned to form a novel material that may be used for suturing and other uses within the body as it will have a definite but slow dissolving time within body tissues.

With the above objects and others in view, the nature of which will be more apparent, the invention will be more fully understood by-reference to the accompanying detailed description and the appended claims.

Thiolated gelatin is prepared by reacting a thiolactone with the free amino groups of gelatin to thereby attach free sulfhydryl or thio (SH) groups to the protein chain through pep-tide bonds. This product exhibits all the known properties of gelatin including high water solubility but, in addition can be made into heat-stable, insoluble and unmeltable gels by oxidizing the sulfihydryl groups to form disulfide cross links. Under proper conditions of pH or added oxidizing agents, a heat-stable, insoluble gel can he formed, either instantaneously or over varying periods of time as may be desired. The resulting lgel becomes insoluble at temperatures in excess of 212 F. and is resistant to dilute acid and alkali.

'Ihiolated gelatin is available commercially in various combinations of bloom number (-350), molecular weight (10,000-100,000) and average SH equivalentper 100,000 g. (6-l5). In order to obtaineificient crosslinking, the thiolated gelatin must be in the gel state (or in some form. of pre-gelatin, the latter being defined as a condition in which the gelatin chains have some degree of orientation. It is known that this orientation exists through aggregation of vthiola-ted gelatin in aqueous sol-utions of at least 2 to 3% :at low temperatures but above freezing. Higher temperatures up to will not disrupt the aggregation in more concentrated solutions, i.e.,

greater than 8%. Thus, oxidation of completely disoriented thiolated gelatin does not lead to a cross-linked product and the closer the gel state approached before oxidation, the higher the degree of cross-linking which may be obtained. I

The oxidizing agent must be selected to provide quantitat-ive oxidation of the sulfhydryls but should not carry the oxidation beyond the disulfide state. Two agents which have been used with excellent results are potassium ferricyanide and atmospheric oxygen inasmuch as excesses do not cleave the disulfide bonds and the K Fe(CN) is excellent for diffusing into a gel mass and causes crosslink-ing almost instantaneously at pH 6.4 or higher. Other more vigorous agents such as peroxides, halides, etc. mus-t Patented Oct. 8, 1963 be used with care as excesses thereof will carry the reaction past the disulfide stage to the sulfonic acid stage.

The rate of oxidation (particularly by atmospheric oxygen) is effected by many known factors such as number of SH groups (increases with greater number), pH, thickness of gelatin film, etc. Moreover, a gel containing a mixture of conventional gelatin (eg 10%) and thiolated gelatin (e.g. 3%) results in a heat-stable, insoluble product after cross-linking of the thiolated gelatin. It is obvious that various ways of effecting cross-linking that are known to the art can be utilized in the present invention.

The invent-ion, as described here in detail, is embodied in corrugated, knitted, bleached-Teflon or Dacron tubes intended for use as arterial grafts but is obviously applicable to any fabric regardless of its composition, shape, formation, etc.

10 g. of Thiogel B" (Schwarz BioResearch, Inc., trademark for thiolated gelatin) is wetted in 50 g. of deaerated cold distilled H 0, 40 g. of additional H O added and the whole heated and agitated until the solution is clear and free from lumps and air. The gelatin solution should be above 2% and up to the maximum that can be retained as a clear solution which is about 15%, although depending somewhat on temperature and molecular weight. The solution is cooled to 85-90 (Fahrenheit used throughout herein) and 3 g. of a plasticizer mix added thereto that includes 25 parts sorbitan trioleate, 25 parts polyoxyethylene sorbitan monopalmitate and 50 parts of 70% sorbitol and stirred until a homogenous milky solution results. This is then cooled to about 80 and maintained for dipping purposes. A solution of 3 g. K Fe(CN) and 5 g. NHA HCO in 92 g. distilled H O at a pH of 8.3 is held at a temperature of 4550 as the cross-linking solution.

Example 1 A knitted, bleached-Teflon tube is dipped in the gelatin solution, then placed in the cross-linking solution for about 15 minutes and then washed for about one hour to insure washing of all K Fe(CN) from the gelatin film. The tube is then dried and is ready for surgical use after autoclaving one or many times as desired or otherwise sterilized. It has a very pliable and effective coating through which blood does not leak and will permit tissue growth through the fabric shortly after insertion as enzymes in the blood will digest the cross-linked thiolated gelatin from the artery wall. Moreover, the tube is neither slippery nor sticky on wetting.

Example 2 A knitted Dacron tube, having been stored in a refrigerator at about 40", is immersed into the dipping solution and returned to the refrigerator to set. This is repeated 3 to 5 times or until a desired thickness of gelatin is obtained. After the last dipping, the tube is held in the refrigerator for at least one hour which permits a portion of the sulfhydryl groups to form disulfide cross-links.

The tube is then placed in the cross-linking solution for about two hours, removed and washed repeatedly in cold H O until all of the yellow color has disappeared.

A tanning solution of 5 g. 2-hydroxymethyl-2-nitro-l,3 propanediol, 5 g. Nl-I HCO and 3 g. sodium acetate in 87 g. of deionized H O having a pH above 10 is held at about 70 and the tube placed therein for one hour. The solution with the tube therein is raised to about 120 and held there for 20 minutes. The tube is ther' removed and placed in cold running H O for 20 minutes, rinsed in distilled H 0 and dried in a warm, dust-free air current.

This tube has the same general properties as Example 1 except that the coating of cross-linked and tanned gelatin will be absorbed much more slowly, due both to the increased thickness and slower absorption per se of the double treated gelatin.

These examples are summarized in the following flow 1'} sheet, steps 1 to 4 being applicable to Example 1 and steps 1 to 6 to Example 2:

(l) Mixing thiolated gelatin and plasticizer Holding artery in crosslinking solution l Washing out unreacted cross-linking material Holding artery in tanning solution Washing out unreacted tanning solution After having tried many materials for the replacement of arteries including homographs, solid tubes, and fabric tubes of all various types including knitting, weaving and braiding, the surgical art has presently accepted only knitted or woven, Teflon or Dacron, tubes. Initially there was an attempt to have the fabric have as low porosity as possible to prevent leakage during the surgical pro cedure but the trend has now turned towards a more porous fabric which leads to quicker and better healing. Thus there is a demand for a temporary coating on the fabric tubes to prevent leakage at the implantation but a coating that is preferably removed from the porous understructure within about 5 to 14 days to promote quick healing and tissue invasion through the porous tube.

Therefore, Example 1 presently satisfies the requirements of many surgeons and takes about one and onequarter hours to manufacture most of which is simple washing, whereas it took at least four hours baking to denature a conventional gelatin coat that could be autoclaved and the coating of Example 1 is vastly superior to the denatured gelatin coat.

Example 2 is merely an example of the other extreme to give a thicker coating, a completely crosslinked coat (requires additional oxidation time primarily because of the increased thickness) and in addtion thereto a tanned coatng, it being understood, of course, that tanning is a different reaction from cross-linking. The amount of coating would, of course, be controlled to a certain extent by the initial porosity of the tube. The product of Example 2 is generally not needed for artery substitution but it does have the advantage that it can be more readily standardized on digestive time per a particular enzyme, or time of absorption in the body. However, as a general rule the surgeons are interested in a coating that will prevent leakage of blood initially and thereafter be absorbed as quickly as possible from the implanted artifical artery.

Any of the various known oxidizing methods may be used for cross-linking of thiogelatin including those previously mentioned, cupric ions, and many others that will be obvious to those in the art. The same is true, of course, for tanning agents which are well-known in the art and include among many others, formaldehyde, formaldehyde producing substances, acetaldehyde, crotonaldehyde, glutaraldehyde, basic chrome sulfate, aluminum salts, titanium salts, iron salts, sulfonyl chloride and syntans such as naphthelene sulfonic acid formaldehyde and phenol sulfonic acid.

The plasticizers as disclosed are desirable, but not essential, to secure a softness in the gelatin coating, the degree of softness being, of course, controlled by the amount of plasticizer and any of the various plasticizers or combinations thereof known to the art may be used that are not toxic, such as glycerines, glycols, sorbitols, mannitols, sugars, and substituted compounds of each of these.

Any degree of cross-linking would, of course, be helpful but generally it is desirable to complete the crosslinking which is easily done herein in Example 1 by the combination of a thin coat of thiolated gelatin and exposure to K Fe(CN) This same coating of cross-linked thiolated gelatin (tanned or untanned) may, of course, be applied to fabrics used in the body for other surgical procedures, e.g., Teflon fabric utilized for cardiac patches where the problem is largely the same as with blood vessels, namely, to prevent leakage during the operation but to have a porosity exposed soon thereafter to promote healing and tissue growth penetration through the fabric. Moreover, it is within the scope of this invention to add heparin (anti-clotting) or thrombin (clotting) to the thiolated gelatin solution before subsequent cross-linking alone or with tanning.

Furthermore, the gelatin product produced by the combination of cross-linking and tanning is a novel material per se and will have many uses both within and without of an animal body, as it :has a very high tensile strength and may be autoclaved and reautoclaved for sterilization. For example, it might be used as a sponge, fibers, substitute for catgut sutures (yarn or filament), powders, solid films, perforated sheets, foam and blocks, to be used both internally and on the skin and solid tubes to be used within the body that are intended to be removed subsequently, te.g., drainage tubes. Other uses will, of course, suggest themselves for this material.

We claim:

1. A fabric coated with a cross-linked by oxidation thiolated gelatin.

2. A fabric blood-vessel graft coated with a crosslinked by oxidation thiolated gelatin.

3. A knitted blood-vessel graft coated with a crosslinked by oxidation thiolated gelatin.

4. A woven blood-vessel graft coated with a crosslinked by oxidation thiolated gelatin.

5. A knitted, polytetrafiuoroethylene blood-vessel graft coated with a cross-linked by oxidation thiolated gelatin.

6. A knitted, polyethylene glycol terephthalate, bloodvessel graft coated lated gelatin.

7. A fabric blood-vessel graft coated with a crosslinked by oxidation thiolated gelatin and a plasticizer for said gelatin.

with a cross-linked by oxidation thio- 8. A fabric blood-vessel graft coated with a crosslinked by oxidation and tanned thiolated gelatin.

9. A modified gelatin product comprising a thiolated gelatin that was cross-linked by oxidation and subsequently tanned.

10. The product of claim 9 additionally comprising plasticizers.

11. The product of claim 9 additionally comprising heparin.

12. The product of claim 9' additionally comprising thrombin.

13. The product of claim 9 in film form.

14. The product of claim 9 in filament form.

15'. A process for impregnating a fabric with a nonmelting, water-insoluble gelatin product comprising coating said fabric with a solution of thiolated gelatin, and cross-linking by oxidation said thiolated gelatin.

16. The process of claim 15 wherein said solution contains a plasticizer.

17. The process of claim is with K Fe(CN) 18. The process of claim 15 wherein said cross-linked gelatin is further tanned.

19. The process of claim 15 wherein said tanning is with 2-hydroxymethyl-2-nitro-1,3-propanediol.

20. A process for forming a non-melting, water-insoluble, solid gelatin product comprising cross linking by oxidation a solution of thiolated gelatin to form a solid and tanning said solid gelatin.

21. The process of claim 20 wherein said solution includes a plasticizer.

22. The process of claim includes thrombin.

23. The process of claim 20 wherein said solution includes heparin.

24. The process of claim 20 wherein said solid is in film form.

25. The process of in filament form.

16 wherein said cross-linking 20 wherein said solution claim 20 wherein said solution is References Cited in the file of this patent UNITED STATES PATENTS 2,072,302 Herrmann et a1. Mar. 2, 1937 2,122,907 Calva July 5, 1938 2,334,098 Hubbard Nov. 9, 1943 OTHER REFERENCES Edwards: Surgery, vol. 45, No. 2, pp. 298-309, February 1959.

Lancet, Oct. 1, 1955, pp. 711 and 71 2. 

1. A FABRIC COATED WITH A CROSS-LINKED BY OXIDATION THIOLATED GELATIN. 